1. Field of the Invention
The present invention relates to a write coil with a submicron pitch and more particularly to a write coil with submicron pitch, low profile and high write current conducting capability.
2. Description of the Related Art
A write head is typically combined with a magnetoresistive (MR) head to form a merged MR head which has sensitive elements exposed at an air bearing surface (ABS). The write head comprises first and second pole pieces that are connected at a back gap, the back gap being recessed from the ABS. The first and second pole pieces have first and second pole tips, each having first and second edges, the first and second edges terminating at the ABS. These edges are sensitive elements of the write head that are exposed at the ABS. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in the insulation stack. A processing circuit is connected to the coil layer for conducting information signal currents (write signals) through the coil layer. The write signals cause the coil to induce corresponding information signal fields on the first and second pole pieces. A magnetically insulative gap layer is sandwiched between the first and second pole tips so that the information signal fields fringe across the first and second edges of the first and second pole tips at the ABS. The read head includes an MR sensor sandwiched between first and second insulative gap layers. This sensor is the sensitive element of the read head that is exposed at the ABS. The first and second gap layers, and the MR sensor, are sandwiched between first and second shield layers. In a merged MR head, the second shield layer and the first pole piece are a common layer. The MR sensor detects magnetic fields of a rotating disk by a change in its resistance that corresponds to the strength of the magnetic fields. A sense current conducted through the MR sensor produces voltage changes that are received by the processing circuitry as readback signals. In a magnetic disk drive a magnetic disk is rotated adjacent to, and a short distance from, the ABS so that the write fields magnetize the disk along circular tracks, thereby storing information in the form of magnetized areas that can be detected by a read head.
There is a strong-felt need to minimize the size of the aforementioned components of the write head in order to increase the data rate of the head. Steps have been taken to reduce the length and thickness of the first and second pole pieces. This decreases the reluctance of the head, and supports higher frequency signals, which equate to higher data rates. Another component affecting the reluctance of the head is the write coil. Typically, the write coil includes one or more pancake-shaped coils stacked on top of each other, and separated by insulation layers. Steps have been taken to reduce the thickness and pitch of the write coil in order to improve the coil's frequency response. The pitch is the lateral spacing between adjacent coil lengths.
The write coil is typically constructed by frame plating. A seed layer is sputter deposited on top of a first insulation layer of the insulation stack. Photoresist is then spun on the seed layer, patterned by light, and then developed to leave a spiral-shaped opening where the coil is to be formed. Material, such as copper, is then electroplated in the opening to a desired height. The photoresist is then removed by a solvent, leaving one pancake-shaped coil. This method permits the coil to be constructed with desired heights, and with a close pitch. The primary problem with this coil is that the edge along its length is vertical, owing to the vertical side walls of the patterned photoresist. It is desirable that the bottom of the coil be flared outwardly, so as provide the write coil with increased write current capability, however, the method does not permit such flaring. Thus, the only way to increase write current with this method is to increase the height of the coil. Unfortunately, this increases the height of the insulation stack. A higher insulation stack causes the following problems: (1) a thicker photoresist layer is manifested in the pole tip region for constructing the second pole tip, which imposes a lower limit on track width; and (2) more light is reflected from the insulation stack into the photoresist layer in the pole tip region, resulting in jagged photoresist edges and a poorly formed second pole tip, after plating. Another problem is that the coil material is essentially limited to copper, since aluminum cannot be plated by this method. A further problem with this method is that plating is a wet process which is more difficult and expensive than the sputtering process that is discussed next.
Another method of making a write coil is to sputter, or plate, a coil material layer. If the coil layer is plated, a seed layer must first be sputter-deposited, followed by plating. Sputtering is a dry process; it may comprise conventional sputtering, or ion beam sputtering. In conventional sputtering, a work chamber contains a plasma ionized with material from a target for deposition on the workpiece. In ion beam sputtering, a gun within the chamber contains the plasma, and the gun directs the ionized plasma to the workpiece. A photoresist layer is then spun on the workpiece and patterned as described hereinabove. Ion milling, which is similar to sandblasting, is then employed to remove a portion of the coil material layer exposed by the patterned photoresist. The coil is finished after removal of the photoresist. A problem with this method is that ion milling results in redeposition on the side walls of the photoresist, which stays in place after removal of the photoresist. Redeposited material appears as a fence above the top of the coil, and it must be removed by some means, such as scrubbing. Another problem with this method is that, upon removing the photoresist, the first insulation layer below the coil, comprising hard baked photoresist, is exposed to the solvent employed for stripping the resist, and may be damaged thereby.
Still another method of constructing the write coil requires a modification of the aforementioned ion milling process, in which a sacrificial layer is formed on top of the coil material layer. Photoresist is patterned on top of the sacrificial layer, and then the sacrificial layer is reactively ion etched (RIE) through a spiral opening in the photoresist, leaving a coil of the sacrificial layer on top of the coil material layer, where the coil material is to remain. The coil material layer is then milled with an ion beam through a spiral opening that extends through the sacrificial layer, down to the first insulation layer. The sacrificial layer can then be left in place, or removed by a reactive ion etch that is fashioned not to attack the coil. We experimented with this method employing Ta as the sacrificial layer. Because of the thickness of the Ta required to be sacrificed during the ion milling step, the Ta layer was relatively thick. This then required that the photoresist employed for shaping the Ta be relatively thick, because of its consumption during the RIE step. During the light exposure step, the scattering of light increased with increased photoresist depth, causing a loss of definition and making it impossible to obtain a coil with a submicron pitch. Another problem with this method was redeposition of material milled from the sacrificial layer on the side walls of the coil. A further problem was that the angle of the ion milling beam to a normal to the plane of the coil material layer was limited because of the shadowing of the relatively high sacrificial layer. This then limited the expansion of the base of the coil layer for the purpose of increasing its write current capacity.